Important Terminology

Relative humidity

The ratio of the partial pressure of water vapor in an air-water mixture to the saturated vapor pressure of water at a prescribed temperature. The relative humidity of air depends not only on temperature but also on the pressure of the system of interest.  This is the ratio expressed as a percentage of the amount of water vapor in the air to the amount, which would saturate it at the same temperature and pressure.

RH = Water Vapor Pressure /  Saturation Water Vapor Pressure x 100%

Air will always contain moisture in the form of water vapor unless specially prepared (temperature reduced to -273°C).  At any particular temperature the air can hold a limited amount of moisture but the higher the temperature the greater its capacity.  When this capacity is reached it is said to be saturated.  If unsaturated air is cooled the temperature will eventually be reached at which it becomes saturated and if the temperature is lowered further still, the air will no longer be able to hold all of the moisture and some will be deposited.  This is said to be the Dewpoint of the gas.

Relative humidity of the atmosphere is the dominating factor in determining whether corrosion or mold growth occurs.  This applies equally to both inside and outside an equipment, the former being most significant.


The presence of water vapor in the atmosphere or gas

Absolute Humidity

This is the unit weight of water vapor present per unit volume of gas.

Water Vapor Pressure

This is the fundamental measure of humidity.  Water vapor in a gas can be treated as a gas itself and behaves in accordance with the standard gas laws.  The total pressure of the gas (Pt) is composed of the partial pressure of the gas (Pg) plus the partial pressure of the water vapor (Pw)

Pt = Pg + Pw

Vapor Pressure

This is sometimes referred to as partial pressure.  This is the part of the total pressure, which is contributed by the water vapor.

PPmV (Parts per million by volume)
PPmW (Parts per million by weight)

This is the number of parts of water to one million parts of dry gas.  Parts per million terminology is generally only used up to about 10,000.

This is not a direct unit reading.  It is calculated from either dewpoint or relative humidity (R.H.)  Take note of the severely NON-LINEAR relationship between dewpoint and PPM scales.

Usable Scales:
It is worth looking at this comparative table to see how when one scale is near the end of its usefulness the others are still usable. (Ex.)

Dewpoint (°F) : -14°F
R.H. (@70°F) : 5%
PPM (V) : 1020

However, it must be remembered should the ambient temperature at which the measurement is being made, then the R.H. value will change but the other two will remain the same because they are measuring absolute humidity.


the effect temperature has upon gases ability to carry moisture is best   illustrated on a standard psychometric chart.  It can be seen that with an increase in temperature a gases ability to carry moisture increases.


The effect pressure has upon a gas, its dewpoint, is best shown on a dewpoint/pressure conversion chart.  See charts.

You will notice that with a change of pressure the dewpoint changes, whereas the PPM remains unchanged.


Leakrates are potentially the easiest way for water vapor to enter an equipment which is essentially sealed.  The rate of diffusion will depend upon: the nature of the leak and its configuration, and the pressure differentials present.

Ideally, to maintain an effective long term sealed system a leakrate of the order of 1 x10-3cc per second or lower should be achieved.  As a rough guide this equates to an equipment being pressurized and completely immersed under water, no bubbles should be evident.

Key points – Temperature stability is vital for accurate results.  Not only could th results be inaccurate but also in extremes it could be possible for water to ingress into the container.  You also have to differentiate between a slight leak and bubbles trapped in any possible holding places on the equipment

Moisture Vapor Transmission Rate (MVTR)

moisture will enter an equipment via gaskets, seals, connectors, and even through the material the equipment is made of.  As a good rule of thumbs, all non-metals have a degree of hygroscopicity and therefore have an MVTR.

MVTR is relative to the partial pressure, which exists across the container or reservoir.  If an equipment has a leakrate of 1 x 10-3cc per second or lower then MVTR is probably going to be the main way water vapor will enter.  This is one of the main considerations when calculating the desiccant life expectancy.

Dew Point

A measure of atmospheric moisture. It is the temperature to which air must be cooled in order to reach saturation (assuming air pressure and moisture content are constant). A higher dew point indicates more moisture present in the air. It is sometimes referred to as Dew Point Temperature, and sometimes written as one word (Dewpoint). PPMv – Parts per million is a close up measure of indicating the amount of moisture within a system.  One part per million (ppm) denotes one part per 1,000,000 parts, one part in 106, 1/1,000,000 * 100% = 0.0001% (or 1% = 10,000 ppm), and a value of 1 × 10−6. This is equivalent to one drop of water diluted into 50 liters (roughly the fuel tank capacity of a compact car) or about 32 seconds out of a year.


desiccant is a hygroscopic substance that induces or sustains a state of dryness (desiccation) in its local vicinity in a moderately well-sealed container.  One measure of desiccant efficiency is the ratio (or percentage) of water storable in the desiccant, relative to the mass of desiccant.  This is typically represented by an adsorption percent by weight.  Another measure is the residual relative humidity of the air or other fluid being dried.  The performance of any desiccant varies with temperature and both relative humidity and absolute humidity. To some extent, desiccant performance can be precisely described, but most commonly, the final choice of which desiccant best suits a given situation, how much of it to use, and in what form, is made based on testing and practical experience.